Internal combustion engines can be configured to operate using a variety of fuels, including diesel, gasoline, natural gas, ethanol, and other suitable fuel types. While fuel combustion converts some chemical energy into mechanical energy, the fuel combustion process is usually sub-optimal and typically produces a complex mixture of pollutants. These emissions often include solid material and undesirable gaseous compounds, such as, carbon-based particulate matter, nitrogen oxides, and sulfur compounds. In response to heightened environmental concerns, regulatory agencies have increased the stringency of emission standards for such engines, forcing engine manufactures to develop systems to further reduce engine emissions.
One method used by engine manufacturers to reduce engine emissions includes the use of particulate filters configured to capture particulate matter produced by the combustion reaction. However, over time the particulate filter may become clogged with particulate matter and may require cleaning through a regeneration process, wherein particulate matter is purged from the filter. Filter regeneration can be achieved using several techniques, such as, for example, reversing gas flow through the filter, removing the filter for a manual cleaning, or raising filter temperature to burn off accumulated particulate matter. The “burn off” process offers a relatively simple and practical solution that includes igniting and burning trapped carbon to form carbon dioxide and/or carbon mono-oxide that can then pass through the filter in gaseous form.
Several strategies can be used to raise filter temperature during a regeneration process. Ideally, the filter temperature and oxygen concentration of the exhaust gas should be sufficient to ignite the carbon matter and continue the burn to completion. Such exhaust gas conditions may be achieved using engine management systems to regulate the combustion process, heating coils located in the exhaust system, or microwave energy applied to the filter.
Filter regeneration can also be achieved using an oxidation catalyst positioned in an exhaust flow upstream of the filter, and configured to raise exhaust temperature to oxidize trapped particulate matter. During regeneration, the oxidation catalyst may operate in conjunction with a fuel supply, wherein the fuel supply provides fuel to the catalyst. The catalyst oxidizes the fuel via an exothermic reaction that increases exhaust temperature. In order to maintain the exothermic reaction, the catalyst material should be maintained at temperatures above a “light-off” temperature, the temperature at which catalyst materials can spontaneously oxidize hydrocarbons. Such light-off temperatures are about 250° C.
One filter regeneration system that includes an oxidation catalyst is described in U.S. Patent Application Publication No. 2005/0241301 (hereinafter “the '301 publication”) of Okugawa et al., published on Nov. 3, 2005. The '301 publication describes an exhaust cleaning system that includes a particulate filter and an oxidation catalyst positioned upstream of the particulate filter. The exhaust temperature can be increased by providing excess hydrocarbons to the exhaust gas and oxidizing the hydrocarbons via an exothermic reaction within the oxidation catalyst to heat and regenerate the particulate filter.
Although the regeneration system of the '301 publication may regenerate the particulate filter, the system includes various sensors and an electronic control unit (ECU) configured to adjust engine performance. The ECU monitors the filter temperature and adjusts various engine operating parameters, such as, air-to-fuel ratio and valve-timing, to periodically modify the combustion process to raise the exhaust temperature, and hence regenerate the filter. Such a system may increase the cost and complexity of a regeneration system, and increase the fuel consumption by operating an engine at less than optimal conditions.
The present disclosure is directed at overcoming one or more of the limitations in the prior art.